The present invention relates to fluid flow fields for fuel cells. More particularly, it relates to flow field designs for supporting structurally weak and/or mechanically anisotropic fluid diffusion layers in solid polymer electrolyte fuel cells.
Fuel cell systems are currently being developed for use as power supplies in numerous applications, such as automobiles and stationary power plants. Such systems offer promise of economically delivering power with environmental and other benefits.
Fuel cells convert reactants, namely fuel and oxidants, to generate electric power and reaction products. Fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. A catalyst typically induces the desired electrochemical reactions at the electrodes. Preferred fuel cell types include solid polymer electrolyte fuel cells that comprise a solid polymer electrolyte and operate at relatively low temperatures.
During normal operation of a solid polymer electrolyte fuel cell, fuel is electrochemically oxidized at the anode catalyst, typically resulting in the generation of protons, electrons, and possibly other species depending on the fuel employed. The protons are conducted from the reaction sites at which they are generated, through the electrolyte, to electrochemically react with the oxidant at the cathode catalyst. The catalysts are preferably located at the interfaces between each electrode and the adjacent electrolyte.
A broad range of fluid reactants can be used in solid polymer electrolyte fuel cells and may be supplied in either gaseous or liquid form. For example, the oxidant stream may be substantially pure oxygen gas or a dilute oxygen stream such as air. The fuel may be, for example, substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or an aqueous liquid methanol mixture in a direct methanol fuel cell. Reactants are directed to the fuel cell electrodes and are distributed to catalyst therein by means of fluid diffusion layers. In the case of gaseous reactants, these layers are referred to as gas diffusion layers.
Solid polymer electrolyte fuel cells employ a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d) which comprises the solid polymer electrolyte or ion-exchange membrane disposed between the two electrodes. Each electrode contains a catalyst layer, comprising an appropriate catalyst, located next to the solid polymer electrolyte. The catalyst may, for example, be a metal black, an alloy or a supported metal catalyst, for example, platinum on carbon. The catalyst layer may contain ionomer which may be similar to that used for the solid polymer electrolyte (for example, Nafion(copyright)). The catalyst layer may also contain a binder, such as polytetrafluoroethylene. The electrodes may also contain a substrate (typically a porous electrically conductive sheet material) that may be employed for purposes of mechanical support and/or reactant distribution, thus serving as a fluid diffusion layer.
The MEA is typically disposed between two plates to form a fuel cell assembly. The plates act as current collectors and provide support for the adjacent electrodes. The assembly is typically compressed (for example, at about 70 psi overall) to ensure good electrical contact between the plates and the electrodes, in addition to good sealing between fuel cell components. A plurality of fuel cell assemblies may be combined in series or in parallel to form a fuel cell stack. In a fuel cell stack, a plate may be shared between two adjacent fuel cell assemblies, in which case the plate also serves as a separator to fluidly isolate the fluid streams of the two adjacent fuel cell assemblies.
In a fuel cell, flow fields are employed for purposes of directing reactants across the surfaces of the fluid diffusion electrodes or electrode substrates. The flow fields comprise fluid distribution channels separated by landings and may be incorporated in the current collector/support plates on either side of the MEA. The channels provide passages for the distribution of reactant to the electrode surfaces and also for the removal of reaction products and depleted reactant streams. The landings act as mechanical supports for the fluid diffusion layers in the MEA and provide electrical contact thereto. Thus, flow fields serve a variety of functions, and appropriate flow field designs involve a balance of the various related requirements in order to obtain satisfactory results overall.
In an effort to improve fuel cell performance and to reduce the thickness and cost of membrane electrode assemblies, there is a trend to use thinner, more porous materials for the fluid diffusion layers. However, these materials tend to be weaker mechanically. In addition, certain mass produced materials having some of these desirable features also have anisotropic mechanical properties relating to the method of production (for example, they have an orientation or grain direction). In operation, some materials used as fluid diffusion layers may delaminate with time (for example, from exposure to water at high temperatures breaking down resins in the material) thereby weakening the layer mechanically. These weaker and/or mechanically anisotropic materials may require more support than that provided by conventional flow field plates in order to prevent the material from deflecting into the flow field channels under the compressive loads applied in the fuel cell stack. If deflection of the diffusion layer material is not prevented, channels become obstructed, thus impairing the distribution of reactants and/or removal of reaction products and adversely affecting fuel cell performance. In addition, deflection of the material can itself result in delamination too.
Simply increasing the landing area and/or the number of similar lands in a flow field design may improve the mechanical support of an adjacent fluid diffusion layer but this also adversely affects fluid access to and from the fluid diffusion layer. Support may however be improved without necessarily increasing landing area. For instance, additional support members may be inserted between the flow field plates and the diffusion layers as disclosed in U.S. Pat. No. 6,007,933. In that patent, the use of support members such as meshes or expanded metals was disclosed in order to provide enhanced stability to the diffusion layers. However, that approach involves the use of additional components which increase cell thickness, complexity, and cost.
Alternatively, improved mechanical support may be provided without adversely affecting fluid access to and from the diffusion layer by using flow fields with smaller, more closely spaced channels such as those disclosed in published PCT patent application number WO 00/26981. In that application, performance results were disclosed for flow fields having channels with inclined walls and with reduced channel and land widths adjacent the diffusion layer of approximately 300 xcexcm and 30 xcexcm respectively. By reducing the span across the flow channel, the xe2x80x9ctentingxe2x80x9d or deflection of soft diffusion layers into the channels may also be reduced. However, the diffusion layer span parallel to the channels can still be relatively large (with straight channels, the span is the length of the flow field). This may still be a concern, particularly if the adjacent diffusion layer is anisotropic with a grain direction parallel to the channels.
A variety of other flow field designs have been proposed in the art for one reason or another that may also provide improved support of mechanically weak fluid diffusion layers. For instance, flow field plates with interdigitated inlet and outlet channels formed in porous plates (for example, as disclosed in U.S. Pat. No. 5,641,586) may provide improved support via relatively large porous land areas in the porous plate. Woven metal meshes might be employed that directly define a rectangular flow field pattern (for example, as disclosed in U.S. Pat. No. 5,798,187). However, again the diffusion layer span parallel to the channels in these designs may be relatively large and the use of such designs may also involve certain other disadvantages.
A flow field design is provided that is capable of supporting mechanically weak and/or anisotropic fluid diffusion layers in a fuel cell while still adequately supplying a fluid reactant to a fluid diffusion electrode comprising such a diffusion layer and adequately removing depleted reactant and reaction products therefrom. Herein, an anisotropic fluid diffusion layer refers to a layer with significant differences in mechanical properties between the two dimensions defining the major surface of a sheet-like fluid diffusion layer. It particularly refers to those layers having relatively high bending strength in one major dimension (for example, parallel to the xe2x80x9cgrainxe2x80x9d) and relatively weak bending strength in another (perpendicular to the xe2x80x9cgrainxe2x80x9d).
The flow field comprises one or more fluid distribution channels separated by landings in which the landings mechanically support the fluid distribution layer. Typically, the flow field comprises a plurality of fluid distribution channels. The channels in the flow field have an average channel width W and are configured such that essentially any unsupported rectangular surface of length L and width W on an adjacent fluid diffusion layer has a ratio L/W less than about 3. Any linear portions in the flow field channels are therefore essentially all less than 3 times the average channel width. (For a flow field channel whose average channel width is less than about 1 mm, a linear portion in a flow field channel therefore has a length of less than about 3 mm.) Any non-linear portions in the flow field channels are configured such that unsupported rectangular surfaces on the diffusion layer are all essentially smaller than 3W by W in size as well. Thus, the diffusion layer surface is reasonably supported in essentially every direction.
Support may be particularly improved over conventional straight channel configuration flow fields (or those comprising channels with significant linear portions) when the ratio L/W is less than about 2. Again, for flow fields whose average channel width is less than about 1 mm, this corresponds to a linear portion in a flow field channel being less than about 2 mm.
A suitable flow field configuration comprises fluid distribution channels that are shaped in a wave form, such as a sinusoidal shaped wave form. Modifications of sinusoidal or similar shaped wave forms may also be used.
The fluid distribution channels may also be cross connected. This may be desirable in order to minimize the effect of any channel blockages (for example, by water reaction product) or to force some fluid flow to occur in the regions of the fluid diffusion layer directly above the landings (by momentum or differences in gas velocity between channels). With sufficient multiple cross connections, the flow field may resemble a pattern of dimples or posts in which the dimples or posts mechanically support the fluid diffusion layer.
The flow fields are advantageous for use in a fuel cell comprising a fluid diffusion electrode which employs a fluid diffusion layer that is relatively weak mechanically or is anisotropic such that it is relatively weak in one major dimension. Weak fluid diffusion layers include those with a Taber stiffness less than about 2 Taber units or that deflect more than about 50 micrometers over a 900 micrometer span under the mechanical loading applied over the fuel cell plates. The flow fields are particularly suitable for use in solid polymer electrolyte fuel cells employing thin, highly porous diffusion layers. Such diffusion layers may comprise carbon fibres and be manufactured in continuous webs in such a way that the fibres become aligned (oriented) in the machine direction and thus the web has a xe2x80x9cgrainxe2x80x9d. As a consequence, such webs may have significantly weaker bending strength perpendicular to the grain.